US 7697414 B2
A system and method for achieving crest factor (CF) reduction for multi-carrier modulation in a wireless communication network (100), such as an ad-hoc peer-to-peer multi-hopping mobile wireless communication network (100). The system and method use the properties of the Inverse Fourier Transform (TFT) for achieving crest-factor reduction. Specifically, the system and method map original signal input frequencies to a new set of frequencies by mapping every input frequency to some other input frequency, and then using the IFT to create multiple versions of the transmitted signal and then computing the transform with the lowest CF and selecting that signal for transmission.
1. A method for performing multi-carrier modulation communication in a wireless communication network, the method comprising operating by a node within the wireless communication network to:
map each of a plurality of input frequencies using pseudo-random mapping to another frequency to create a plurality of randomly ordered mapped frequencies;
perform a mathematical operation on the mapped frequencies to create transformed frequencies;
determine a respective crest factor (CF) value for each respective transformed frequencies; and
select for transmission a signal having the transformed frequency that has a reduced crest factor (CF) value that meets a condition.
2. A method as claimed in
the mapping, mathematical operation and selecting steps are performed by a controller and a transceiver of the node.
3. A method as claimed in
applying by the node a phase reference to each of the respective input frequencies before mapping each of the respective input frequencies.
4. A method as claimed in
the mathematical operation includes a Fourier transform operation.
5. A method as claimed in
the Fourier transform operation includes one of an Inverse Discrete Fourier Transform (IDFT) operation and an Inverse Fast Fourier Transform (IFFI) operation.
6. A method as claimed in
the node stops performing the Fourier transform operation on the mapped frequencies when the node determines that a transformed frequency has a CF value that meets the condition.
7. A method as claimed in
the condition is a value below a predetermined threshold CF value.
8. A method as claimed in
the threshold CF value is variable.
9. A method as claimed in
modulating by the node each respective signal before performing the mapping step.
10. A method as claimed in
the communication network includes a wireless ad-hoc communication network; and
the method further comprises operating the node to transmit the selected signal in the wireless ad-hoc communication network.
11. A node, operating to perform multi-carrier modulation communication in a wireless communication network, the node comprising:
a frequency mapping device, operating to map each of a plurality of input frequencies using pseudo-random mapping to another frequency to create a plurality of randomly ordered mapped frequencies, to perform a mathematical operation on the mapped frequencies to create transformed frequencies, to determine a respective crest factor (CF) value for each respective transformed frequencies, and to select for transmission a signal having the transformed frequency that has a reduced crest factor (CF) value that meets a condition.
12. A node as claimed in
the frequency mapping device includes a controller, operating to create the mapped frequencies; and
a transceiver, operating to transmit the selected signal.
13. A node as claimed in
the frequency mapping device further operating to apply a phase reference to each of the respective input frequencies before mapping each of the respective input frequencies.
14. A node as claimed in
the mathematical operation includes a Fourier transform operation.
15. A node as claimed in
the Fourier transform operation includes one of an Inverse Discrete Fourier Transform (IDFT) operation and an Inverse Fast Fourier Transform (IFFT) operation.
16. A node as claimed in
the frequency mapping device stops performing the Fourier transform operation on the mapped frequencies when a transformed frequency is identified having the CF value that meets the condition.
17. A node as claimed in
the condition is a value below a predetermined threshold CF value.
18. A node as claimed in
the threshold CF value is variable.
19. A node as claimed in
the frequency mapping device operates to modulate each respective signal before mapping the frequency of each respective signal.
20. A node as claimed in
the communication network includes a wireless ad-hoc communication network, and a transmitter operates to transmit the selected signal in the wireless ad-hoc communication network.
The present invention relates to a system and method for achieving crest factor reduction for multi-carrier modulation in a wireless communication network, such as an ad-hoc peer-to-peer multi-hopping mobile wireless communication network, using the properties of the Inverse Fourier Transform.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks,” issued on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel,” issued on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System,” the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, radio frequency (RF) power amplifiers are used in wireless communication networks for the transmission of signals. Solid state RF power amplifiers can be modeled by amplitude-modulation/amplitude-modulation (AM/AM) characteristics, because the amplitude-modulation/phase-modulation (AM/PM) characteristics are negligible. As the input signal amplitude to amplifier is increased, the output signal will begin to saturate at some level in the RF amplifier. The amplitude where the output signal moves from the linear region to the saturation region is usually referred to as the one decibel (1 dB) compression point of the amplifier.
Different models for RF power amplifiers have been developed, and an important feature of each of these models is the manner in which the output signal moves from the linear region to the saturation region. At the extreme end of the model amplifiers are the completely linear model and limiter model. To maintain linear operation, power amplifiers are usually backed off by some number of dBs from the one decibel (1 dB) compression point. The required back-off depends on the crest factor (CF) of the input signal. For orthogonal frequency division multiplexing (OFDM) modulation, back-offs higher than 5 dB are generally used. Typical back-off numbers are in the region of 9-12 dB for 64 OFDM.
As understood in the art, the crest factor (CF) of a signal can be defined as the peak to average amplitude ratio. The peak to average power ratio (PAPR) can be computed from the CF. High CF multi-carrier modulation, such as OFDM, can pose problems for RF power amplifiers because they require high linearity. To maintain linearity, power amplifiers are usually backed-off from their 1 dB compression point so that they can reproduce the high peak powers of the signal without distortion.
The high CF of OFDM results from the individual carrier components being added together at different phases in the Inverse Fast Fourier Transform (IFFT). The CF is relatively independent of the modulation method of the individual carriers when there are many sub carriers. The CF is also relatively independent of the number of sub carriers for practical 32-256 sub carrier OFDM modulations.
If the CF of the signal can be reduced prior to the RF power amplifier, the back-off can be reduced and mean output power can be increased. Numerous CF reduction methods have been developed. Generally, these include selective mapping where multiple information equivalent signals are created and the lowest CF signal is then selected for transmission. Partial transmit signal is a similar approach where multiple partial signals are generated and the most beneficial linear combination is transmitted. In addition, hard and soft clipping methods limit CF by removing or reducing peaks.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to a system and method for achieving crest factor reduction for multi-carrier modulation in a wireless communication network. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of a system and method for achieving crest factor reduction for multi-carrier modulation in a wireless communication network described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method for achieving crest factor reduction for multi-carrier modulation in a wireless communication network. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.
As will now be described, the present invention provides a system and method that can achieve crest factor reduction without requiring windowing, multiple partial transmitted signals, clipping in a soft or hard manner, nor the application of multiplicative signals or kernels to phase shift the sub carrier components.
As can be appreciated by one skilled in the art, the nodes 102, 106 and 107 are capable of communicating with each other directly, or via one or more other nodes 102, 106 or 107 operating as a router or routers for packets being sent between nodes, as described in U.S. Pat. Nos. 7,072,650, 6,807,165 and 6,873,839, referenced above.
As shown in
Each node 102, 106 and 107 further includes a memory 114, such as a random access memory (RAM) that is capable of storing, among other things, routing information pertaining to itself and other nodes in the network 100. As further shown in
As will now be described, the embodiments of the present invention described herein provide a system and method that is capable of using the beneficial properties of the Inverse Discrete Fourier Transform (IDFT) to achieve crest factor reduction for multi-carrier modulation in a wireless communication network, such as an ad-hoc peer-to-peer multi-hopping mobile wireless communication network. The controller 112 in cooperation with the memory 114 and transceiver 108 of any of the nodes as shown in
Therefore, if the original input frequencies are mapped to a new set of frequencies by mapping every input frequency to some other input frequency, the IDFT will create a new output sequence. By creating multiple mappings, different versions of the same input signal can be generated by IDFT. Hence, this technique does not require the application of multiplicative kernels or phase rotations as it uses the properties of IDFT.
The CF reduction system and method further uses these properties by generating a map 300 having L maps where each input frequency is mapped to another input frequency as shown in
The OFDM modulator 400, which can be located in the transceiver 108 along with the other components shown in
It is also noted that the number of IDFTs that are evaluated can be reduced by stopping the process when a CF value is computed that is lower than a threshold. The threshold can be variable depending on the success of previous symbols as the average number of evaluations determines the use of resources. The disclosed technique thus uses multiple IDFTs or IFFTs. Some of the input signals are the same for all frequency-to-frequency maps. This feature can be used for optimized IDFT or IFFT by not clocking the logic with unchanged inputs.
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.